Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Semiconductor devices perform a wide range of functions such as analog and digital signal processing, sensors, transmitting and receiving electromagnetic signals, controlling electronic devices, power management, and audio/video signal processing. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, diodes, rectifiers, thyristors, and power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, application specific integrated circuits (ASIC), power conversion, standard logic, amplifiers, clock management, memory, image sensors, interface circuits, and other signal processing circuits.
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual images for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
FIG. 1 illustrates a conventional leadless package 110 as a quad flat no-lead (QFN) or dual flat no-lead (DFN) package. Leadless package 110 includes a semiconductor die 124 disposed on a leadframe 126. Leadframe 126 is formed from a metallic substrate with material of the substrate removed to create a plurality of leads 126a surrounding a die pad 126b. Semiconductor die 124 is disposed on die pad 126b and includes contact pads coupled to leads 126a by bond wires 136. An encapsulant or molding compound 140 is disposed around semiconductor die 124 and leadframe 126 for electrical isolation, physical support, and protection from contaminants.
Leadless package 110 is mounted to a printed circuit board (PCB) or other substrate 120. Solder 150 is reflowed between leadframe contacts 126a and contact pads 122 on PCB 120 to form a metallurgical and electrical connection between leadless package 110 and the PCB. Leadless package 110 includes leads 126a for external interconnection, which are simply portions of a metal leadframe exposed from the final package. Leads 126a are used instead of leads that extend from the package laterally and/or vertically as in traditional semiconductor package types. The exposed wettable material of contacts 126a on the lateral surfaces of package 110, referred to as a wettable flank, allows solder 150 to form filleted surfaces 152 after leadless package 110 is mounted onto PCB 120.
Fillets 152 are useful to manufacturers of electronic devices because proper interconnection between package 110 and PCB 120 can be verified visually by a human or by an automatic visual inspection device 156 including an image sensor and a computer programmed to analyze the images. If a visual inspection shows that a proper fillet 152 was not formed for one of the connections of a lead 126a to a contact pad 122, an error in the specific PCB 120 is recorded. The device with the faulty connection can be repaired or discarded. If visual inspection device 156 verifies that each connection between leadless package 110 and PCB 120 includes a proper fillet 152, the manufacturer can have confidence that the package is properly connected to the system as a whole.
Leadless package 110 reduces the footprint required on PCB 120 over many prior art packages by not having leads that extend from the package, and instead having leads 126a that remain within the footprint, and within the height, of the package body. However, forming fillets 152 for visual inspection relies on side surfaces of leads 126a being wettable by solder 150. That is, solder 150 should flow up the sides of leads 126a to form a fillet 152 between side surfaces of contacts 126a and contact pad 122.
To promote reflow of solder 150 onto side surfaces of contacts 126a, a manufacture of package 110 will commonly plate leads 126a with a wettable material as part of the manufacturing process of the package. Metal plating is commonly done with various electroplating or electroless deposition methods. Electroplating is the more desirable plating method because a thicker and faster metal deposition occurs. The thicker metal layer provided by electroplating provides a more reliable solder fillet, especially after parts sit on a shelf for a significant period of time before being soldered. On the other hand, electroplating for the full thickness of contacts 126a can be a challenge because a portion of the side surfaces may remain during electroplating to provide electrical current for the electroplating process.
Therefore, a need exists for leadframe designs that are capable of providing electrical interconnection to leads for electoplating the full thickness of the leads, while also allowing subsequent electrical isolation of the leads for use of the final package in a larger system.